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On 2018-03-20 20:15, Terry Syrokosz wrote:
 > Spring occured at 16:17:19.333... GMT  03/20/18

On 2018-03-21 8:30, Dave Walden wrote:
>From JPL, I make it:
> 16:15:21.750...UTC


I put the equinox (the conventional beginning of spring) at 16:15:27
UTC. In chronological order:

16:15:20 UTC zero right ascension
16:15:27 UTC zero ecliptic longitude
16:16:04 UTC zero declination
16:16:24 UTC zero ITRS latitude

The second line (zero longitude) is the formal definition of the
equinox. If the Sun were exactly on the ecliptic the first three
phenomena would occur at the same instant. However, the Sun oscillates
north and south of the ecliptic on a monthly cycle. At this equinox its
ecliptic latitude is -.7″. By happenstance that's practically the
southern peak of the oscillation.

The last line is the layman's definition of the equinox. For most people
that's close enough, and avoids an explanation of ecliptic coordinates.

Zero ITRS latitude does not coincide with zero declination because
Earth's axis of rotation (the basis of the celestial equator) is a few
tens of feet from the ITRS (geodetic) pole. Therefore the planes of the
celestial and geodetic equators fail to coincide by a few tenths of an
arc second.

My computations of the Sun's geocentric apparent place utilzed the JPL
DE431 ephemeris, IAU 2006 precession, and 2000B nutation models. Pole x
= 0.0036″, y = 0.3775″ (these affect only the time of zero ITRS latitude).

The JPL Horizons online calculator says zero ecliptic longitude occurs
about 1.5 seconds after my time. That's due to a difference in
precession / nutation models. To duplicate the Horizons values, 1) do
NOT apply frame bias, 2) use the IAU 1976 precession and 1980 nutation
models, 3) to the nutation angles add the celestial pole offsets
dEps1980 and dPsi1980 published by the IERS.

   

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